(1) Field of the Invention
The invention described herein relates to a modem system using business or multiline type telephone stations for sending and receiving data at through-put rates that exceed those of conventional or single telephone line modem systems.
All present day modems, regardless of the protocol or modulation technique used, employ only one telephone line to transmit and receive data. Multiline business type telephone stations utilize a plurality of telephone lines so that if one of the lines is in use, i.e. busy, the next "free" line can be seized. If, for example, two stations each have a three line business telephone system and all three lines from both stations are not in use, then a multiline modem system connected at the originating end operating at 9600 bits per second (bps) on each telephone line could transmit data at the equivalent rate of 28,800 bps. Using the transmission rate cited previously, a 30 Megabyte data file could be transmitted using a multiline modem in 2.5 hours rather than 7 hours at the single line rate of 9600 bps.
(2) Description of the Prior Art
The switched telephone network was designed years ago for the primary purpose of voice or analog transmission between two stations. This network utilizes a "local loop" that connects a subscriber telephone to a central office which supplies the necessary switching, signaling and power equipment. The local loop comprises twisted pair wires that connect the telephone to the central office. This twisted pair wire limits the upper frequency response of the local loop to about 3500 Hz. Transformers at the central office limit the lower frequency response of the local loop to about 300 Hz. This finite bandwidth of approximately 3000 Hz limits the rate of information throughput that the network can process.
The present day telephone network utilizes a number of transmission methods to connect one subscriber telephone to another. The local loop to the central office is the primary connection from the "originating" telephone to the "answering" telephone. Alternate transmission methods such as microwave links and special toll connecting trunks are used to carry voice information beyond the network capabilities of the central office. Since the local loop to the central office is the only actual physical or "wire" connection, transmission of digital or computer data over telephone lines must be in a form suitable with all elements in the telephone network.
Modems, or Modulators/Demodulators are systems that change the form of digital computer data into a form suitable for transmission over the telephone network. A modem does not operate or alter the content of the computer data; it merely changes the form of the data to be compatible with the telephone network. One of the first methods employed to modulate digital computer data was Frequency Shift Keying (FSK). The first FSK modems were full duplex 300 bits per second (bps) devices that encoded the computer's "ones" and "zeroes" into two distinct frequencies of the originating computer and two other distinct frequencies for the answering computer. This type of encoding allows one bit of information to be encoded per baud where one FSK modem transmits at 300 bps. FSK modems generally cannot exceed 1200 baud because of the telephone network's bandwidth limitations mentioned previously.
Newer methods of encoding digital information for transmission over telephone networks have been devised, most notably differential phase shift keying (DPSK) and quadrature amplitude modulation (QAM). DPSK and QAM modems, unlike FSK modems, can encode more than one bit per baud. Thus a DPSK modem operating at 1200 bits per second which encodes 2 bits per baud is actually transmitting at 600 baud. DPSK modems differ from FSK modems in that the phase of a constant frequency analog signal is varied to encode the data. QAM modems vary the amplitude of the transmitted analog signal as well as its phase. QAM modems typically encode up to 4 bits per baud. A QAM modem operating at 9600 bps which encodes 4 bits per baud would be transmitting digital data at a 2400 baud rate.
Computers generally contain a number of input/output (I/O) ports for communicating with external devices. These I/O ports can be parallel or serial in form. A serial, or asynchronous, I/O port is the connecting interface for external modems. When the computer's serial port outputs digital data, it "frames" the serial data with start and stop bits. A typical serial data stream from the computer would consist of one start bit, eight data bits, and one stop bit. Each data bit can be a binary one or a zero, and start and stop bits are usually fixed as ones or zeroes. A modem connected to the computer's serial output port would convert these ones and zeroes into an FSK, QAM or DPSK form for transmission over the telephone network. At the receiving end of the network, a modem of the same type would accept these modulated signals and convert them into the digital computer ones and zeroes for input into the receiving or answering computer's serial input port. The receiving or answering computer can also output serial data from its serial output port to its modem for transmission over the telephone lines to the originating computer's modem. The originating computer's modem, in addition to transmitting data to the answering computer, can also accept data from the answering computer's modem. This method of simultaneous transmission and reception by both computers using the switched telephone network is known as full duplex communication.
A simple example of a modern day modem's limited throughput capability will illustrate the need for a new and innovative method for transmitting and receiving digital data over conventional telephone lines. A computer with a 30 megabyte (thirty million bytes of information where one byte is equal to eight bits) storage capacity is connected to a modem and the entire contents of the computer's memory will be transmitted at a speed of 9600 bps to a remote computer. If one byte is equal to one character of information, then the total time to transmit 30 M bytes of data would exceed 7 hours. A method of improving data throughput over conventional telephone lines is needed in order to reduce the data transfer time from one computer to another.